1. Field of the Invention
The present invention relates generally to an apparatus and method for combining symbol data in a CDMA (Code Division Multiple Access) communication system, and in particular, to an apparatus and method for combining symbol data with a fading component removed.
2. Description of the Related Art
Generally, a base station in a CDMA communication system separates a transmission data signal d(t) into I and Q signals by 1:2 demultiplexing the data signal d(t) into an even-numbered data signal and an odd-numbered data signal. The I and Q signals are multiplied by a corresponding orthogonal code for orthogonal spreading and a pilot signal is added only to the I signal for complex pseudo-noise (“PN”) spreading. The resulting signals pass through filters and are upconverted in frequency prior to transmission. A mobile station receiver receives the signal on a fading channel from the base station. The input signal at the receiver can be expressed asS1(t)=(PW0+IWd+jQWd)(PN(I)+PN(Q))×(cos(wt)+jsin(wt))(f(I)+jf(Q))  Eq. 1where (f(I)+if(Q)) is a fading component, P is a pilot signal, W0 is a pilot channel orthogonal code, Wd is a predetermined orthogonal code for orthogonal modulation and PN(I) and PN(Q) are orthogonal code for orthogonal spreading.
Therefore, the mobile station combines multi-path signals and demodulate symbol data with the highest energy to achieve the best reception. As a result, a decoder can perform accurate decoding. It has been reported that a maximum ratio combining method results in the highest performance. In the maximum ratio combining method, input symbol data is multiplied by a fading component for channel compensation and only symbol data with energies higher than a predetermined level among the channel-compensated symbol data are combined.
The problem with the maximum ratio combining method is that the fading component is entirely canceled when multiplying the symbol data by the fading component only when the phase of received symbol data is equal to the phase of the fading component. Accurate phase compensation to eliminate the fading component has not been achieved in the prior art. Therefore, symbols are combined while the fading component exists. Possible combination of symbols with a deep fading component reduces symbol energy, thereby nullifying the effect of symbol combination to obtain a maximum energy.